1. Field of the Invention
The present invention relates to communication protocols and more particularly to packaging of information transmitted over a SONET compatible network.
2. Description of the Related Art
In the field of communications, it is critical to provide a high bandwidth and reliable, low latency delivery of information such as video, data, and voice. Synchronous Optical Network (SONET) is a well known standard for high bandwidth transmission that uses optical carriers. SONET is defined in ANSI T1.105, ANSI T1.106, and ANSI T1.117, which are herein incorporated by reference. For a discussion of SONET, see also xe2x80x9cSonet and T1: Architectures for Digital Transport Networksxe2x80x9d by Ulysses Black and Sharleen Waters, Prentice Hall, ISBN 0-13-447590-9, which is incorporated herein by reference in its entirety. In Europe, a standard similar to SONET, called Synchronous Digital Hierarchy (SDH) is used.
SONET defines a transmitted unit called xe2x80x9csynchronous payload envelopexe2x80x9d. FIG. 1 depicts a synchronous payload envelope for Synchronous Transport Signal Level 1 (STS-1). In STS-1, a synchronous payload envelope is expressed as a 9 row by 90 column structure with 810 bytes total. Columns 1 to 3 correspond to a transport overhead, column 4 corresponds to a path overhead, and columns 4 to 90 correspond to an associated payload. The transport overhead and path overhead together provide operations, maintenance, provisioning, and administration information data channels within the SONET system. The transport overhead and path overhead are discussed in more detail below. The payload stores information such as video, data, or voice, each potentially packaged in various formats.
FIG. 2 depicts the layout of format bytes within the section, line, and path overheads in more detail. The format bytes in the section, line, and path overheads are well known in the field of SONET. Of particular interest are the bytes Z3, Z4, and Z5 of the path overhead. Those bytes have been denoted xe2x80x9cgrowthxe2x80x9d bytes under the SONET standard, thereby remaining unspecified and allowing for use as seen necessary.
SONET requires synchronization at three levels: bit, time slot, and frame. Bit synchronization refers to the transmission and receipt of bits with respect to a master clock frequency. Time slot synchronization refers to the alignment of transmitter and receiver so that time slots can be identified for retrieval of data. Time slot synchronization is accomplished by using a fixed frame format that separates overhead and payloads. Frame synchronization is the requirement that the beginning of each payload can be identified according to the master clock frequency. Frame synchronization further includes identifying the beginning of each frame/packet payload within the SONET payload according to the master clock frequency. For a general discussion of synchronization in SONET, see xe2x80x9cSynchronizing Telecommunications Networks: Basic Conceptsxe2x80x9d, Hewlett Packard Application Note 1264-1, April 1995 and xe2x80x9cSynchronizing Telecommunications Networks: Synchronizing SDH/SONETxe2x80x9d, Hewlett Packard Application Note 1264-2, July 1995 which are incorporated herein by reference in their entirety.
Bit errors in transmitted packets or frames can lead to loss of synchronization, which can result in the discarding of packets or frames. One source of bit errors is improper frame/packet delineation, i.e., improper determination of a frame or packet boundary. Another source of bit errors is a noisy communications link.
Multi-protocol over ATM (MPOA) and PPP-over-SONET (POS) are two well known protocols for packet packaging and transport over SONET. For a description of MPOA, see xe2x80x9cMultiprotocol Encapsulation over ATM Adaptation Layer 5xe2x80x9d by Heinenen, R.F.C. 1483, July 1993, which is incorporated by reference herein in its entirety. For a description of POS, see xe2x80x9cPPP over SONET/SDHxe2x80x9d, RFC 1619, W. Simpson, May 1994, which is incorporated by reference herein in its entirety and provided in Appendix A. One disadvantage of MPOA is its requirement that packetized information be segmented and reassembled to perform Layer 3 IP routing at each intermediate node. This requirement potentially slows the delivery of MPOA packetized information, leading thus to unsatisfactory delivery of video or other time critical information. One disadvantage of POS is that bit errors due that cause loss of synchronization increase with average packet size. As multimedia applications tend to have ever larger packet sizes, the reliability of POS potentially decreases for multimedia applications such as streaming video.
A method and system are disclosed for packaging frames or packets encoded in a variety of formats, for example, Frame Relay, Ethernet, ATM, or TCP/IP. The method includes: accumulating first and second packets; determining the length of each packet; and storing the lengths into a header associated with the first packet. By using the length field, frame synchronization can be efficiently achieved and maintained for the packets encoded using the method.
Various embodiments of the present invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings.